1. Field of the Invention
The present invention relates to a gap controller for keeping constant the gap between a solid immersion lens (SIL) for producing near-field light and an optical information storage medium that has been loaded into an optical information processor to read and/or write a signal from/on the storage medium using the near-field light. The present invention also relates to an optical information processor including such a gap controller and to a method for driving the optical information processor.
2. Description of the Related Art
A technique for further increasing the data storage density of an optical disc by using an SIL and near-field light has been proposed. A typical SIL is a high-refractive-index lens, of which the shape is defined by cutting out a portion of a spherical lens. An SIL is inserted between a condenser lens and an optical disc to produce near-field light at the bottom.
To read and write data from/on an optical disc, which is an exemplary optical information storage medium, using an SIL, the SIL and the side of the optical disc (which will be referred to herein as a “signal read/write side”) needs to be so close to each other that the near-field light can reach the signal read/write side of the optical disc. In addition, by performing a so-called “gap control” to keep the gap between the SIL and the optical disc surface constant, the light beam spot on either the disk surface or the information storage layer should maintain a predetermined size.
A read/write operation using an SIL is disclosed in detail in U.S. Pat. No. 6,496,468, the entire disclosure of which is hereby incorporated by reference.
Such a gap control is carried out by taking advantage of the property that the intensity of the light returning from an SIL changes its levels according to the magnitude of the gap between the SIL and the given optical disc. Japanese Patent Application Laid-Open Publication No. 2002-319160 discloses an exemplary optical information processor that performs such a gap control. Specifically, such a gap control is carried out by comparing a voltage converted from the intensity level of the light returning from the SIL to a reference voltage representing a desired gap and by getting the SIL driven by an actuator such that the difference between these two values becomes as small as possible. The magnitude of the gap can be varied by changing the reference voltages.
To produce near-field light between the SIL and the signal read/write side of the given optical disc, the gap between them should be adjusted to an extremely small value of 20 nm, for example. And such a target gap value should remain the same even if the reflectance of the given optical disc has changed.
If the two given optical discs have mutually different reflectances, however, their gap-returning light intensity level characteristics are also different from each other as shown in FIGS. 4 and 5. In this case, FIG. 4 is a graph showing the relation between the gap and the returning light intensity level in an optical disc with a reflectance of 10%, while FIG. 5 is a graph showing the relation between the gap and the returning light intensity level in an optical disc with a reflectance of 25%. In both of FIGS. 4 and 5, the abscissa represents the gap (nm) and the ordinate represents the returning light intensity level, which is shown as a ratio when the returning light intensity level associated with an infinite gap is supposed to be one. That is why the returning light intensity level when the magnitude of the gap is equal to zero represents the reflectance itself.
If the given optical disc has the characteristic shown in FIG. 4, the gap control should be carried out with the gap control reference level defined to be 0.27 in order to maintain the gap at 20 nm as described above. On the other hand, if the given optical disc has the characteristic shown in FIG. 5, the gap control reference level should be 0.39 to maintain the same gap.
As is clear from this fact, to carry out a gap control on various optical discs with mutually different reflectances such that the gap value remains the same, the gap control reference levels should be redefined appropriately. That is to say, when an optical disc from/on which data is going to be read or written is loaded into an optical disc drive, a returning light intensity level associated with the desired gap needs to be known in advance. For that purpose, an optical information processor such as an optical disc drive would normally need to include both                1. means for measuring the returning light intensity level, and        2. means for measuring the gap accurately.        
The returning light intensity level can be measured with a photoelectric transducer, for example. However, since the desired gap has an extremely small value of about 20 nm, it is difficult in the current state of the art to realize means for setting such a small gap exactly and build such means in an optical information processor. Also, even if such means for setting the gap exactly were realized, that means would be a rather bulky device. That is why it is far from being realistic or beneficial to introduce such a bulky device into an optical disc drive as a consumer electronic product.
In order to overcome the problems described above, the present invention has an object of providing a gap controller that realizes the desired gap even if the given optical information storage medium has a different reflectance from the previous one.
Another object of the present invention is to provide an optical information processor including such a gap controller.